It's Saturday night. Of your group of 6 + 1, you have only 2 players and the DM. Or maybe 4 and no DM. What do you do?
Call the session off? Push on?
Back in the wayback times, when work and family didn't prevent us from wasting a Sunday afternoon at the dining room table playing Chaos Marauders, Talisman, Nuclear War, Sega Genesis and the like, the session went one, but not the campaign.
That is a major advantage for face to face gaming, as the session (if not the campaign) can easily continue with a core chunk of players missing.
These days I play pretty much exclusively online via G+ Hangouts and Roll20. We did try, briefly, having an alternate system (T&T) when we had less than the optimal number of players, but that didn't work so well (might be that I'm probably the only member of the group well versed in T&T).
Board games are out. Drinking Quest would probably work, but each person on would probably need their own set of the rules.
For the most part, these undermanned sessions turn into amazing chat sessions, but then again, I do game with some very creative people.
So, what do you do on the game nights where you don't have enough peeps to play in the regular RPG campaign?